In modern military communications, radar, and electronic warfare, so-called "exotic" signals are now common, as are very dense signal environments. Real time processes for signal detection and analysis, interference cancellation and timing acquisition are therefore becoming increasingly important and computationally intensive. Optical signal processing, due to its parallel structure and the natural implementation of fundamental signal processing algorithms such as the Fourier transform, offers one of the more promising and successful techniques for wide-band signal processing. The disadvantage of optical signal processing lies in the electronic to optical conversion and optical to electronic interfaces, which create significant bottlenecks in the process.
Acousto-optic Bragg cells represent the most successful technology to date for the electronic/optical interface in signal processing. These Bragg cells have evolved to become the premier device for data input to broad band with optical signal processing systems. Bandwidths ranging from 20 MHz to in excess of 1 GHz are presently available with time bandwidth products in the range of 1,000. Recent activity in optical signal processing has focused on using photorefractive materials as a potential photodetector/processor element.
Photorefractive materials have temporal and spatial response characteristics which make them well-suited to adaptive filter architectures based on time integrating correlator configurations with acoustic Bragg cell input devices. A significant advantage of the photorefractive integrator approach is that it is readily extended to two-dimensional processing by using arrays of acoustic channels. This makes the approach potentially very effective for adaptive antenna array processing.
An adaptive filter architecture using a photorefractive element and a time-integrating structure to compute correlation coefficients has been described by J. Hong, S. Hudson, J. Yu, D. Psaltis, in "Photorefractive Crystals as Adaptive Elements in Acousto-optic Filters", SPIE Vol. 789 Optical Technology from Microwave Applications III, Orlando, 1987. The optically-computed correlation coefficients are simultaneously used to optically form a signal estimate and adaptive correlator. This processor represents a two-stage optical computing process in which it is not necessary to convert to electrical signals between a computation of the correlation coefficients and their subsequent use.
The above-described system is relatively large due to the fact that separate Bragg cell arrays are used for computing correlations, and for performing the final signal weighting and summation. There are thus two separate paths, one for the "write" beam and one for the "read" beam. In applications where space is a critical factor, such as in aircraft, the use of separate beam paths for the read and write beams and separate Bragg cells for the two functions of computing correlations and final weighting makes the apparatus less desirable.
There is a need for a photorefractive adaptive filter that is both rugged and compact, yet provides a fast response with low power usage. Such an adaptive filter can be used in a phased array antenna, for example.
Phased array antennas have many benefits when compared to fixed beam antennas including the ability to form multiple beams, the ability to scan rapidly without mechanical motion, and the ability to perform pattern nulls on interfering emitters. The implementation of real arrays which realize these advantages is limited by the complexity of the phase shift network required by unpredictable phase errors in the components involved. Adaptive techniques have the potential for alleviating many of these problems.
Successful systems to date have relied on discrete RF implementation of the adaptive algorithms with small arrays (small because of the complexity and expense of the necessary hardware) using analog or digital implementation of the required amplitudes and phases. Optical techniques have been used to perform the calculations, but the systems employed to date have been large, complex, and limited in performance because of their complexity.
There is a need for a correlating weighting structure that can be used in an adaptive antenna array processor that is compact, rugged and operates with a fast response using low power.